Facilitated transport of hydrogen sulfide

ABSTRACT

H+MN+BBM-A=NM+B+HBM(1-A)   WHERE NB=MA. SALTS PROVIDING ANY OF CO3=, HCO3-AND PO4= ARE EMPLOYED BY WAY OF EXAMPLE. IN AN IMMOBILIZED LIQUID MEMBRANE THE TRANSPORT OF AN ACID GAS, E.G. HYDROGEN SULFIDE, WHICH IS CAPABLE OF DISSOLVING IN WATER AND DISSOCIATING, IS FACILITATED BY THE ADDITION OF WATER-SOLUBLE SALT    MN+BBM-A AND/OR MN+BHBM(1-A)!   TO THE AQUEOUS SYSTEM, THE ANION OF WHICH REACTS REVERSIBLY WITH H+ AS FOLLOWS:

United States Patent OF THE DISCLOSURE lnan immobilized liquid membrane the transport of an acid gas, e.g. hydrogen sulfide, which is capable of disolvingin waterand dissociating, is facilitated by the addition of water-soluble salt to the aqueous system, the anion of which reacts reversibly with H+ as follows:

where rib;ma.Salts providine ny. O (30 HcO iand- P0 are employed by way of example.

BACKGROUND OF THE INVENTION This invention is an extension of the application ofthe phenomenon of facilitated transport in liquid membranes. The application of facilitated transport to the separation of gases bydiiferential permeation through liquid, or quasi-liquid films (e.g. liquid impregnated microporous sheets) isadescribed in US. Pat. 3,396,510 Wardet al. (incorporated herein by reference) wherein additive species for the facilitated transport separation" of carbon'dioxide', sulfur dioxide and oxygen gases are described. I

It would be desirable to discover a simple and economical treatment for the removal of hydrogen sulfide from gas'mixtures. 7 1".

SUMMARY OF THE INVENTION The symbols employed herein are defined as follows:

M+ Cation of valence 'b.

B' Polybasic'anion of valance a.

HB Polybasic anion'of'valence la.

n Number of cations present in the salt molecule.

m Defined'by the relationship nb=ma.

T he-qtransport ofan acid gas (defined as a gas that will dissociate in an aqueous solution liberating H through an immobilized liquid membrane is facilitated. by the addition of a water-soluble salt [M B -e and/or M HB to an aqueous liquid membrane, the

anion of M B e reacting reversibly with H+ asfollows:

+i -agh ni -m tpai The facilitated-transport of hydrogen' 'sulfide through an aqueousmembraneinto which salts providing specific ions have been introduced-"is described herein.

' 3 Claims ice " DESCRIPTION OF THE PREFERRED EMBODIMENT The permeability of a non-porous permselective membrane, e'.'g; an immobilized liquid membrane (ILM), to a non-reacting .gaseous substance is the product of the solubility and the diffusivity of that gas in the membrane material. Using known values of diffusivity and solubility .for.H S in water at (Chemical Engineers Handbook, 3rd edition, McGraw-Hill, Inc. [1950], pages 540, 675) the permeability of an aqueous ILM to H 8 may be calculated and has a value of approximately:

Hydrogen sulfide dissolves in water to a very appreciable extent. At C. and a partial pressure of one atmosphere of H 5, the concentration of H 3 in water is 0.1 moles/ liter. However, the dissociation of H 8 in water is very limited. The dissociation of H 5 proceeds according to the following reaction:

H S=H++HS- (Reaction 1).

At 25 C. with 0.1 mole/liter of H 8 in solution, the concentrations of H+ and HS will each be 10* moles/liter. This very small HS- concentration is not sutficient to provide significant facilitated transport of H 8 through an-ILM.

According to the instant invention, by forming an ILM from an aqueous solution containing specific anions, the aid tov HS- transfer so provided tremendously enhances the transport of H 8 across the membrane. Similar ILMs may be prepared for the transfer of other acid gases, e.g. HCN, phenol.

Thus, considering an aqueous solution containing a salt (represented by the general expression M B whose anions are capable of reacting as follows:

when H 8 dissolves in this solution, HS- ions will be created according to the following reactions:

With such an ILM (prepared from a solution of M B located so as to separate a region of high H S partial pressure( on the feed gas side) from a region of low H 8 partial pressure (on the sweep gas side), the facilitated transport of H 8 across the membrane occurs primarily by the following mechanisms:

1. H 8 from the feed gas stream dissolves in the solution of salt M B at the membrane surface;

2. H 8 reacts with B according to Reaction 3 to produce HB and HS;

v 3. H 3, HS- and HB diifuse across the membrane to the sweep gas side; 4. at the sweep gas side of the membrane, HS and HB Q- react to produce H 8 and B 5. B g diffuses back across the membrane to the feed gas side; and v 6. H 8 desorbs from the ILM into the sweep gas stream.

'1 here are several restrictions on the selection of the salt M B which must be present in the ILM. These restrictions are:

1. The concentrations of all species in the lILM must be maintained at less than their respective solubility limits -to prevent precipitation of any salts in the membrane. 2.];The salt M B must be very soluble so that sufiiftljlo nlBhflOIls are available (-i.e. in the case of H 8 containing potassium phosphate/ potassium biphosphate in solution, and ILM containing potassium carbonate/potassium bicarbonate in solution and (to establish the pronounced enhancement obtained'in the flux of HS- ions) an ILM containing lithium chloride in solution.

Table I shows the average conditions encountered during the experimental runs (salt concentrations represent initial compositions of the ILMs, which are not'necessarily the final steady state concentrations): 1

TABLE I Membrane 1.5M C Ofl 2M Poi- 1.5M HC 0;- 6M LiCl Membrane area, em. 1 90 119 126 Membrane thickness, cm 00121 0. 0121 0. 0121 Duration of run, hours 144 40 System temperature, C -28 20-22 .20-22 Solution relative humidity, percent 87 85 83 Feed stream:

1. Flow rate, cc.lsec 9.7 10 10 2. Relative humidity, percent =92 =90 =88 3. Inlet H 3 concentration, percent 0. 28 0. 0.30 4. Outlet H S concentration, percent 0. 08 0. ()8 =0. 30' Sweep stream:

1. Flow rate, ccJsec 11.3 11.4 10. 8 2. Relative humidity, percent =91 =90 3. Inlet HIS concentration, pereent 0. 0 0.0 0.0 4. Outlet H S concentration, percent 0. 17 =0. 15 0. 01

4. Finally, the reaction between the acid gas and the B,,;

ions -(e.g. Reaction 3 in the case of H 8) must be completely reversible, and the kinetics must be such that the reaction will be very fast in both the forward and reverse directions.

Experiments with H 8 permeation through ILMs containing various salts have established that by introducing into the ILM the facilitated transport mechanism of the instant invention, the H 8 permeability can be increased at least 75 times the permeability of H 8 through a nonfacilitated ILM.

Such ILMs may be prepared by adding to a liquid B and/or HB wherein these anions are either inorganic or organic. Of the inorganic anions tested the phosphate, carbonate and bicarbonate anions have been found very useful for the facilitated transport of H 8. The potassium salts of these ions are preferred, because of their high solubility. Organic salts useful for facilitated H 8 transport are the potassium salt of diethylglycine and the potassium salt of dimethylglycine. In the case of ILMs for H 8 transport the pH should be in the range of from about 9 to about 11.

In one embodiment, the ILM comprises a matrix layer having interconnecting micropores, which pores are occupied by the aqueous solution. The microporous matrix must, of course, be made of a hydrophilic material (i.e. wet by the particular solution employed). Examples of commercially avail-able microporous materials useful for this purpose are those sold under the trademarks Solvinert and Acropor. ILMs may, of course, be packaged as, for example, by the method shown in US. 3,574,8l9 Neulander et al. (incorporated by reference).

Another embodiment of this invention would support the HM on a hydrophobic microporous membrane;

Solutions used to make ILMs for the practice of this invention were prepared from distilled water and reagent grade K CO K PO or a mixture of K CO and KHCO In the case of the CO /HCO system, it is necessary to maintain a proper carbon dioxide partial pressure-in the feed gas stream in contact with the membrane to pre- The measured permeability for the potassium phosphate/ biphosphate ILM was about cc. H S(STP)-cm.

Observed values of permeability for the potassium carbonate/ bicarbonate ILM were in the range of about These permeation constants are believed to be the high-' est values ever achieved for a non-porous membrane and establish that this transport mechanisrn truly diffusion limited, rather than reaction rate limited as is the case for facilitated O0 ransport as described in US. 3,396,510.

Table 11 displays a comparison of for a number of different membranes.

Salt concentrations represent initial compositions of theLILMs, which are not necessarily the final steady state concentrations.

Lithium chloride does not exercise any facilitation function in H S transport (the requisite reaction between H 8 and Cl" does not occur), and since (as seen in Table II) the measured value of H 8 permeability therethrough I,

Was

-9 430 X 10 ,-cm. -em. g

it may be concluded that the H 8 flux in the potassium vent substantial depletion of the HCQ, ion, which is n required in the system to accomplish proton (H+) transfer across the ILM. i

All feed gas streams were humidified to proper water vapor content by passing them through water-filled saturaters, which almost completely saturated the gas. This precaution was taken in order to prevent drying out of the ILMs. Experiments were conducted with an ILM phosphate/potassium biphosphate and potassium carbonate/potassium bicarbonate systems was in each case tremendously enhanced by the increased flux of HS ions. The vast difference between the values of permeability indicated for the various membranes also confirms that for practical purposes the entire H S flux in the facilitated transport ILM may be considered to be due to the HS- ion transport.

In the case of the'ILMs of this invention, all of the reactions 'oc'cur instantaneously whereby the net H 8 transport is difiusion-controlled and not reaction rate controlled.

permeability to H S In the case of the carbonate/bicarbonate ILM, the average distribution of HCO and CO will be determined by the average partial pressure of CO on both sides of the film and this average value of partial pressure of CO should be such that the concentration of H00 and CO will be of the same order of magnitude (e.g. CO partial pressure of the order of 0.01 atmosphere at 23 C.). Due to the H 8 pressure dilference across the film, significant concentration gradients in HCO and CO are established in the ILM and, fortunately, these gradients will not be altered significantly by the presence of CO adjacent the membrane, because the hydrolysis of CO is a slow reaction relative to the speed of diffusion in the system.

In the absence of CO in the feed gas mixture, one must either add CO gas to the system as indicated above or employ ditferent facilitating species, e.g. the phosphate/ biphosphate system.

What we claim as new and desire to secure by Letters Patent of the United States is:

20 1. In a method for the separation of a given gas from a mixture of gases by bringing the mixture into contact with one side of an immobilized liquid membrane under a partial pressure diiferential in said given gas across said membrane, the liquid of said membrane being impregnated with a soluble non-volatile carrier specie reversibly chemically reactive with said given gas and productive of a non-volatile material soluble therein, the improvement comprising:

(a) said mixture of gases containing hydrogen sulfide and (b) the liquid of said membrane being an aqueous solution containing a water soluble salt selected from the group consisting of the potassium salt of dimethylglycine, the potassium salt of diet hylglycine and inorganic salts providing anions selected from the sub-group consisting of phosphate, carbonate, bicarbonate and mixtures thereof. 2. The improvement of claim 1 wherein the pH is in the range of from about 9 to about 11.

3. The improvement of claim 1 wherein the mixture of gases contains hydrogen sulfide and carbon dioxide and separation is effected therebetween.

References Cited UNITED STATES PATENTS 3,396,510 8/1968 Ward III et al. -16 2,157,879 5/1939 Zublin 423-223 EARL C. THOMAS, Primary Examiner US. Cl. X.R. 

